localtime, _localtime32, _localtime64

Articolo
09/01/2008

Convert a time value and correct for the local time zone. More secure versions of these functions are available; see localtime_s, _localtime32_s, _localtime64_s.

struct tm *localtime(
   const time_t *timer 
);
struct tm *_localtime32(
   const __time32_t *timer
);
struct tm *_localtime64(
   const __time64_t *timer 
);

Parameters

timer
Pointer to stored time.

Return Value

Return a pointer to the structure result, or NULL if the date passed to the function is:

Before midnight, January 1, 1970.
After 03:14:07, January 19, 2038, UTC (using _time32 and time32_t).
After 23:59:59, December 31, 3000, UTC (using _time64 and __time64_t).

_localtime64, which uses the __time64_t structure, allows dates to be expressed up through 23:59:59, December 31, 3000, coordinated universal time (UTC), whereas _localtime32 represents dates through 03:14:07 January 19, 2038, UTC.

In Visual C++ 2005,localtime is an inline function which evaluates to _localtime64, and time_t is equivalent to __time64_t. If you need to force the compiler to interpret time_tas the old 32-bit time_t, you can define _USE_32BIT_TIME_T. Doing this will cause localtime to evaluate to _localtime32. This is not recommended because your application may fail after January 19, 2038, and it is not allowed on 64-bit platforms.

The fields of the structure type tm store the following values, each of which is an int:

tm_sec
Seconds after minute (0 – 59).
tm_min
Minutes after hour (0 – 59).
tm_hour
Hours after midnight (0 – 23).
tm_mday
Day of month (1 – 31).
tm_mon
Month (0 – 11; January = 0).
tm_year
Year (current year minus 1900).
tm_wday
Day of week (0 – 6; Sunday = 0).
tm_yday
Day of year (0 – 365; January 1 = 0).
tm_isdst
Positive value if daylight saving time is in effect; 0 if daylight saving time is not in effect; negative value if status of daylight saving time is unknown. If the TZ environment variable is set, the C run-time library assumes rules appropriate to the United States for implementing the calculation of daylight-saving time (DST).

Remarks

The localtime function converts a time stored as a time_t value and stores the result in a structure of type tm. The long value timer represents the seconds elapsed since midnight (00:00:00), January 1, 1970, UTC. This value is usually obtained from the time function.

Both the 32-bit and 64-bit versions ofgmtime, mktime, mkgmtime, and localtimeall use a single tm structure per thread for the conversion. Each call to one of these routines destroys the result of the previous call.

localtime corrects for the local time zone if the user first sets the global environment variable TZ. When TZ is set, three other environment variables (_timezone, _daylight, and _tzname) are automatically set as well. If the TZ variable is not set, localtime attempts to use the time zone information specified in the Date/Time application in Control Panel. If this information cannot be obtained, PST8PDT, which signifies the Pacific Time Zone, is used by default. See _tzset for a description of these variables. TZ is a Microsoft extension and not part of the ANSI standard definition of localtime.

Nota

The target environment should try to determine whether daylight saving time is in effect.

These functions validate their parameters. If timer is a null pointer, or if the timer value is negative, these functions invoke an invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, the functions return NULL and set errno to EINVAL.

Requirements

Routine	Required header
localtime	<time.h>
_localtime32	<time.h>
_localtime64	<time.h>

For additional compatibility information, see Compatibility in the Introduction.

Example

// crt_localtime.cpp
// compile with: /W3
/* This program uses _time64 to get the current time 
 * and then uses localtime64() to convert this time to a structure 
 * representing the local time. The program converts the result 
 * from a 24-hour clock to a 12-hour clock and determines the 
 * proper extension (AM or PM).
 */

#include <stdio.h>
#include <string.h>
#include <time.h>

int main( void )
{
        struct tm *newtime;
        char am_pm[] = "AM";
        __time64_t long_time;

        _time64( &long_time );           // Get time as 64-bit integer.
                                         // Convert to local time.
        newtime = _localtime64( &long_time ); // C4996
        // Note: _localtime64 deprecated; consider _localetime64_s

        if( newtime->tm_hour > 12 )        // Set up extension.
                strcpy_s( am_pm, sizeof(am_pm), "PM" );
        if( newtime->tm_hour > 12 )        // Convert from 24-hour
                newtime->tm_hour -= 12;    //   to 12-hour clock.
        if( newtime->tm_hour == 0 )        // Set hour to 12 if midnight.
                newtime->tm_hour = 12;

        char buff[30];
        asctime_s( buff, sizeof(buff), newtime );
        printf( "%.19s %s\n", buff, am_pm );
}

Tue Feb 12 10:05:58 AM

.NET Framework Equivalent

System::DateTime::ToLocalTime

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